class Student extends Person {
        constructor(name,age,klass){
        this.name = name;
        this.age = age;
        this.klass = klass;
    }
    introduce(){
        var string  = 'My name is ' + this.name + '. I am ' + this.age + ' years old.'
                        + ' I am a Student. I am at Class ' + this.klass + '.';
        return string;
    }
}
